"C" Shaped Building
"C" Shaped Building
(OP)
I am studying a "C" shaped building. From the exterior, the building appears to be a rectangular box, 50' wide and 120' long. Along the 120' length, the back wall is solid 8" CMU and along the front wall, 60' of the 120' is solid 8" CMU. The 50' wall to the left is 8" CMU, but to the other end of the building, this 50' width is open (storefront). I do not believe this is a stable design, as nothing prohibits the building from racking at the opposite end of the building from the 50' solid CMU wall.
Do you guys agree?
Do you guys agree?






RE: "C" Shaped Building
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it
RE: "C" Shaped Building
RE: "C" Shaped Building
What you have is a 3-sided torsion box - nothing wrong with that.
RE: "C" Shaped Building
Open front structures with rigid wood diaphragms resulting in torsional force distribution are permitted provided the length, l, of the diaphragm normal to the open side does not exceed 25 feet (7620 mm), the diaphragm sheathing conforms to Section 2305.2.4, and the l/w ratio [as shown in Figure 2305.2.5(1)] is less than 1.0 for one-story structures or 0.67 for structures over one story in height. Exception: Where calculations show that diaphragm deflections can be tolerated, the length, l, normal to the open end is permitted to be increased to a l/w ratio not greater than 1.5 where sheathed in compliance with Section 2305.2.4 or to 1.0 where sheathed in compliance with Section 2306.3.4 or 2306.3.5." The AF&PA SDPWS 2008 is similar to this.
Garth Dreger PE
AZ Phoenix area
RE: "C" Shaped Building
RE: "C" Shaped Building
RE: "C" Shaped Building
The building is in a 100 mph wind zone. The roof is comprised of metal roof deck that is screw fastened. I do not know the deck fastening pattern, although I can about guess its something like a 36/4 pattern with (3) sidelap fasteners per span.
The load in the roof diaphragm is 38 kips, based on the 1991 code under which the building was built. This results in a force along the end 50' shearwall of about 760#/Ft (not including any global rotational forces due to torsion).
To begin with, the metal deck cant develope the 760#/ft diaphram capacity. Secondly, even if it could, I believe this existing design is very poor in that it allows the opposite end of the building (opposite the 50' shearwall) to be too flexable....unfortunately, I dont know that that movement can be calculated....but may just be a "feel" sort of thing. Also, there is NO direct connection of the metal deck to the 50' shearwall....the ONLY thing connecting the roof framing to the 50' shearwall is the joist bridging....which takes place (4) times along the 50' wall.
RE: "C" Shaped Building
RE: "C" Shaped Building
Yes to this:
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No to this:
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RE: "C" Shaped Building
And like I say, the connection to the west wall.....there is no connection....except for the joist bridging termination, which I dont count as developing the diaphragm capacity.
Worse yet, the building owner is telling me, "well, its been there 18 years, thru Hugo and a tornado...and now you tell me its no good? etc etc" I told him all I can do is report the numbers...its objective, not subjective.
Also, if I have to retrofit this building....I think it'll have to meet the 2006 IBC, not the 1988 SBCCI...which will require the CMU to be reinforced due to seismic....which cant be done, as the top of the existing wall has a bond beam at +16' that will prohibit the existing walls from being reinforced.....and I dont think I can reinforce the CMU walls externally, as the reinforcing requirements to meet the IBC code minimums for seismic are intended to create ductility in an otherwise brittle system. I fear I may have to tell the client the building will need to come down.
RE: "C" Shaped Building
RE: "C" Shaped Building
Build a model out of cardboard or at least do a cutout of the roof and place it over the plan and see how the edges move off the foundation. You will see that before they can warp, the walls have to fail in in-plane horizontal shear.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: "C" Shaped Building
My calcs reveal the wall will not come close to working, as tensile stress in the block/mortar/faceshell, when subject to lateral wind, is about 107 psi...allowable per ACI 530 is 25 psi. Compressive stress is ok.
The problem here is that the local building official is telling me that this building will have to meet the requirements of the current code now....not the 1988 code.
I believe I'm stuck here.
RE: "C" Shaped Building
RE: "C" Shaped Building
RE: "C" Shaped Building
When you said the roof is metal deck, screw fastened (rather than welded), to me that suggested a metal roof cladding rather than a structural diaphragm deck. If this is the case, with fasteners through the crowns, you can't use the roofing as a diaphragm.
You could probably reinforce the walls by cutting slots, inserting lapped bars in sections, and grouting. You would have to saw out and chip away sections of the bond beam as well. If all the parapet cores are not filled, you could use the empty ones to make the reinforcing continuous.
Not a nice task you have there. The building official may be doing his job now, but the ball was dropped when the building was built.
RE: "C" Shaped Building
I would like to know how the exciting building looks. with a fully unreinforced wall, and no cores grouted, the hand of god must be holding it up. Are you sure we aren't missing something like steel column in the walls, these wouldn't be able to be located with the old tapping method, or something just as weird. Maybe you have a building not build on normal customs, sucht hat there are strange things going on, like maybe the walls have reo in them, but were never grouted.
As for the Shape of the c I have no problem either way as long as the walls and roof can take the loads, and deflections are controlled.
Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it